Photovoltaic devices convert incident solar photons directly into useful electrical work. These devices allow for power generation in remote locations, making them particularly attractive for space power applications. Photovoltaic devices are the primary technology currently used to provide power to space-based payloads.
Space power solar cells have three key performance/design criteria:
(1) Specific power (W/kg): Space deployment costs are the primary expense involved in bringing new satellites online. These costs are governed by the satellite mass, a large fraction of which is the solar power system. Specific power describes power delivered per unit mass and hence determines the power available for satellite payloads.
(2) End of life performance: Radiation exposure in the harsh space environment rapidly degrades solar cell performance. Device designs which maintain solar energy conversion efficiency in these environments allow for reduced mission costs and new extended mission profiles.
(3) Satellite form factor: Conventional space solar cells in accordance with the prior art are implemented in rigid panels. Fully flexible solar panels would allow for new satellite form factors that can be particularly suitable for use in satellite applications. For example, rollable sheets of solar arrays can provide protection and efficient launch stowage of the array and can enable dynamic control of the array's deployment. Flexible solar panels can also enable the deployment of disaggregated micro-satellite swarms without the need for a vulnerable centralized power unit, or can be conformally wrapped around a satellite to provide power generation from every surface.
Current space power solar panels incorporate III-V multi-junction designs such as the third-generation triple junction (ZTJ) solar cell manufactured by SolAero, an exemplary embodiment of which is illustrated in the block schematic shown in FIG. 1. As illustrated in FIG. 1, a typical conventional ZTJ solar cell currently used in space power solar panels is grown on an active Ge substrate 101 with GaAs middle junction 102 and InGaP upper junction 103. This design provides a beginning-of-life (BOL) solar energy conversion efficiency of 29.5%.
However, this conventional cell design has several limitations.
First, conventional solar cells having this design produce relatively low specific power. Since specific power is power per unit mass (W/kg), the thick Ge substrate and radiation-protective cover glass substantially increase the mass of the solar cell, thus reducing the cell's specific power.
Second, conventional solar cells have poor radiation tolerance, which produces poor end-of-life performance because the cells degrade with total radiation dose. Multijunction ZTJ cell devices are particularly sensitive to radiation exposure because the cells are connected in series and hence degradation in any of the subcells limits the performance of the whole stack. Such devices therefore require a protective coverglass over the cells to mitigate this degradation and extend device lifetime. The protection provided by thicker coverglass, however, must be traded against the increased mass and the resulting reduced specific power described above.
Finally, such cells must be implemented in thick, rigid panels, giving them a cumbersome form factor that can limit their use with new satellite designs.
One solution to the problems of ZTJ cells that has been proposed in the prior art uses the inverted metamorphic (IMM) device design illustrated by the block diagram in FIG. 2. Like the ZTJ design, the IMM device design also uses a Ge substrate with a GaAs middle junction and InGaP upper junction. However, in the IMM design, the device is grown in an inverted geometry, with the InGaP top junction 203 grown directly on the Ge substrate followed by the GaAs middle junction 202. A graded buffer layer 204 is then grown to move to a larger lattice constant to enable growth of a low defect InGaAs bottom cell. The structure is then inverted so that InGaAs layer 205 is at the bottom and Ge substrate 201 is at the top of the structure. Ge substrate 201 is then removed by a substrate removal process, leaving a triple junction device.
This structure addresses some of the limitations of the ZTJ design. Removing the heavy Ge substrate reduces the mass of the device, thus providing increased specific power and improving the structure's form factor flexibility. However, the radiation sensitivity of the multijunction cells used in the IMM design still necessitates the use of a coverglass, limiting the device's flexibility and its maximum potential specific power.